fix(ci): drop --max-concurrent-uploads (not a valid docker push flag)

That flag only exists for docker buildx / docker buildx imagetools.
docker push in this version rejects it as unknown. Revert to plain
docker push with the retry loop.

.gitea/workflows/ci.yml

@@ -93,7 +93,7 @@ jobs:
             local max_attempts=5
             while [ $attempt -le $max_attempts ]; do
               echo "=== Push (attempt $attempt/$max_attempts): $ref ==="
-              if docker push --max-concurrent-uploads=1 "$ref"; then
+              if docker push "$ref"; then
                 return 0
               fi
               echo "::warning::Push of $ref failed, retrying in 30s..."